In a wafer fabrication facility, scanning using an optical system is one of the recognized methods for inspecting wafers for defects, irregularities, etc. The scanning approach irradiates all or a specific region of the wafer, for instance in a die or cell on the wafer, and measures one or more parameters of the returning radiation, which may be scattered, diffracted, and/or reflected radiation. The measured parameters may be compared with other respective, assumed “standard” parameters, typically in a cell-cell or die-die comparison, or in a comparison against previously determined values, to determine if the irradiated region is within specification.
The smallest features of elements of present day wafers typically have dimensions of the order of tens of nanometers. Systems for characterizing wafers at these orders of magnitude are known in the art, for example scanning electron microscopes (SEMs), scanning X-ray microscopes (SXMs), atomic force microscopes (AFMs), and Optical Critical Dimension (OCD) tools. However the scan rate of such systems is typically extremely low so that they are usually only used to characterize or inspect relatively small fractions of a wafer. If they are used on a whole 300 mm wafer, the procedure takes many hours or days.
U.S. Pat. No. 6,862,491 to Levin et al., whose disclosure is incorporated herein by reference, describes a method to extend the process monitoring capabilities of a semiconductor wafer optical inspection system so as to be able to detect low-resolution effects of process variations over the surface of a wafer at much higher sensitivity than heretofore possible.
A paper titled “Novel inspection technology for half pitch 55 nm and below” by Omori et al, in Metrology, Inspection, and Process Control for Microlithography XIX, edited by Richard M. Silver, in Proc. of SPIE Vol. 5752, 2005 is incorporated herein by reference. The paper relates to a system for inspecting surfaces.
The following U.S. patents and patent applications, all of which are incorporated herein by reference, relate to systems for inspecting surfaces: U.S. Pat. Nos. 6,512,578, 6,693,293, 7,027,145, 7,248,354, 7,298,471, 7,369,224, 7,372,557, 2004/0239918, 2006/0098189, 2006/0192953, 2006/0232769, 2007/0046931, 2008/0094628.
However, notwithstanding the systems at present available, an improved method for inspecting surfaces is desirable.